A soundproof cover is used for the purpose of cutting the transmission of noise. Typically, the soundproof cover is formed of a nonporous material to have a plate-like shape, and is installed at a position between a target noise reduction region and a sound source so as to be perpendicular to the arrival direction of the noise.
Since the noise has a property of propagating so as to wrap around an object, if a significant noise reduction effect is required, it is common to install the soundproof cover so as to surround the periphery of the noise reduction region, and typically, surfaces with a hexahedral structure having upper and lower, right and left, former and latter surfaces are formed in view of a construction property and cost reduction.
In the case in which the soundproof cover has a single layer, the noise reduction performance of the soundproof cover is basically proportional to the weight thereof, and increases 6 dB every time the weight doubles. In contrast, if a multilayer sandwich structure with an organic porous material sandwiched inside is adopted, it is possible to effectively improve the noise reduction performance with little weight. On the other hand, by adopting the multilayer structure, the installation space and the cost are increased. In general, in the field of acoustic engineering, there is known the fact that there exists a resonant frequency depending on the shape of a flask-shaped container due to the air vibration in the mouth portion of the shape of the container. This is called the Helmholtz resonance principle, and there is a technology of absorbing sound using this principle. For example, Patent Literature 1 discloses a porous soundproof structure for absorbing noise using the Helmholtz resonance principle.
Further, the soundproof cover is also required for a device used in a clean room. In a charged particle radiation device, for example, miniaturization of the pattern of the observation object or the test object has progressed in recent years, and high-magnification observation using electron beams or the like is performed. However, the wobble of an image occurs due to the minute sound pressure or vibration from the outside, and problems due to the vibration have become obvious with the increase in magnification. In such a case, if the soundproof cover using a fiber material such as glass wool or an organic porous material represented by a foam material such as urethane is adopted, a dusting property due to the droplets of the organic material hinders the dust-proof property of the clean room to cause a problem in some cases. Patent Literature 2 discloses a technology of covering the sound absorbing material with dust-proof fibers and then attaching it to an exterior cover.